The instant invention relates to a process control installation for mechanical elements such as pumps, valves, pulleys, control rod release stops, etc. More particularly, the invention may be utilized especially for controls in nuclear power plants, but is in no way limited to this application.
It is known to utilize a process control installation for nuclear power plants in which a predetermined number of physical characteristics are detected or measured, each by several, (e.g., three independent sensors or probes), so that the measurement of each physical characteristic gives rise to a set of several (in this case three), measurement values.
This known installation comprises, first, comparator circuits, each connected to one of the sensors and a reference transmitter, and each delivering at its output a signal the value of which is the difference between the value of reference and the value measured by the probe. Secondly, the installation comprises circuits for the reproduction of signals of the same value as the input signal, galvanically separated one with respect to the other and with respect to the input signal. Each reproduction circuit is connected to the output of a comparator circuit. In the third place, majority decision circuits are provided, each one associated with a physical characteristic. These majority decision circuits are connected to an output of each of the reproduction circuits which in turn receive signals from the comparator circuits connected to the probes measuring or detecting the same physical characteristic. If each physical characteristic is measured by three independent probes each majority decision circuit comprises three inputs. It is further possible to provide "two of three" majority decision circuits the output of which is not dependent on a single breakdown, but is sensitive to a double or triple breakdown. In the fourth place, the installation comprises two identical functional logic flow circuits at one or several outputs of and connected to the output of a majority decision circuit. These logic circuit trains may be very complex and comprise a variety of outputs designed to activate the different mechanical elements of the process control. Thus, in the fifth place, functional logic sets or circuits, each one connected to two corresponding outputs of the two circuit logic trains and influencing, as needed across the power circuits, one or several mechanical process control elements.
It is useful at the point to briefly discuss the concept of the functional logic sets. This set is considered not in view of the output control signal, but in view of its final influence on the functioning of the process. For example, when the anticipated function is the opening of water valves in the event of a fire, if at least one of the logic circuit trains signal the beginning of a fire, a functional OR logic circuit can either open one valve by virtue of an electronic OR logic circuit, or an AND electronic logic circuit so that the signal to open the valve indicates the presence or absence of a fire. According to an alternative, the functional logic OR circuit can activate two valves disposed in parallel to open them, or open one of the two if one alone, or both, if the two logic circuit trains transmit the fire alarm.
The known control installation, described hereinabove is highly reliable in normal operation, however, not being equipped with circuits for intrinsic security, partially loses this quality when one or the other of the two logic circuit trains is tested. Logic circuits for intrinsic security are described, for example, in the French Pat. Nos. 1,410,561, 1,461,822, 1,515,044 and 1,520,105. Furthermore, as is generally the case, when one of the logic circuit trains breaks down, the condition of the output represents the intervention of an urgency measure, for example, the sprinkling in case of a fire. This sprinkling can be initiated by a breakdown of a logic circuit train and may lead to the unnecessarily soaking of a costly installation. The same inconvenience results during an unexpected halt of a chemical process necessitating thereafter the cleaning of the polymerization tunnels and a delicate start up.